1. Field of the Invention
The present invention relates to an interference device. for detecting a positional displacement of an object, a position detection device for detecting position information of an object such as a magnetic head of a hard disk drive, using the interference device, an alignment device, and a servo signal write device using the alignment device.
The present invention is suitably applied to a manufacturing apparatus of a hard disk drive (to be abbreviated as an HDD hereinafter) used in a computer and, more particularly, to a device for accurately writing a servo track signal on a hard disk in the HDD.
2. Related Background Art
FIG. 1A is an explanatory view of a conventional device for writing a servo track signal on a hard disk in an HDD. Referring to FIG. 1A, a hard disk drive HDD has a hard disk RD, slider SLID, magnetic arm ARM1, voice coil motor VCM, spindle OHD (rotation Center) of the hard disk HD, and rotation axis O of the magnetic head arm ARM1.
A magnetic recording medium is deposited on the surface of the hard disk HD. The hard disk HD always rotates at high speed about the spindle OHD, and a magnetic head is placed in the vicinity of the surface of the hard disk HD. The magnetic head is built in a nearly parallelepiped portion called the slider SLID, which is attached to the distal end of the magnetic head arm ARM1 having the rotation center O outside the hard disk HD. When the magnetic head arm ARM1 is rotated by the voice coil motor VCM, the magnetic head can radially relatively move above the hard disk HD.
With this arrangement, the rotating hard disk HD and the magnetic head which moves along an arcuated path can read/write magnetic information at an arbitrary position (track) on the surface of the disk-shaped hard disk HD.
In the magnetic recording method on the surface of the hard disk HD, the hard disk HD is divided into a plurality of annular tracks which are concentric to the rotation center OHD of the hard disk and have different radii. Each annular track is divided into a plurality of arcs. Finally, information is time-sequentially recorded on/reproduced from a plurality of arcuated areas in the circumferential direction.
As a recent trend, an increase in recording capacity of the hard disk is required to record information on the hard disk at higher density. In order to record information on the hard disk at higher density, a method of decreasing the width of each concentrically divided track to increase the recording density in the radial direction is effective.
The recording density in the radial direction is expressed by a track density TPI (track/inch), and is currently around 10,000 TPI. This means that the track spacing is about 3 microns. In order to detect such small track pitch, the magnetic head must be aligned at a resolution (0.05 microns) around {fraction (1/50)} the track width in the radial direction to write a servo track signal in advance. To do this, it is important to devise a technique that can sequentially write servo track signal while aligning the magnetic head at high resolution within a short period of time.
FIG. 1A also depicts a conventional alignment device used for writing a servo track signal. The device shown in FIG. 1A comprises a push rod PROD, an arm ARM2 for the push rod PROD, an alignment control motor MO, a rotary encoder RE for detecting the rotational amount of the rotation shaft of the motor MO, a signal processor SP1 for analyzing the detection output from the rotary encoder RE, and generating an alignment command signal of the magnetic head to the write position of a servo track signal, and a motor driver MD for driving the motor MO in response to the command signal from the signal processor SP1.
Conventionally, as shown in FIG. 1A, the cylindrical surface of the push rod PROD is pressed against the side surface of the magnetic head arm ARM1, and the magnetic head is aligned while sequentially stepping the magnetic head arm ARM1 via the push rod PROD by rotating the arm ARM2 using the motor MO under the feedback control by the system including the rotary encoder RE, signal processor SP1, and motor driver MD. In this way, a servo track signal from a signal generator SG is sequentially written at each position. At this time, in order to assure contact, a current is slightly supplied to the voice coil motor VCM to also press the push rod PROD from the head arm ARM1 side. However, further improvements in performance upon writing information such as servo track signals at high density are demanded by realizing more accurate alignment of a rotary positioner system RTP in consideration of vibrations upon rotation of the hard disk HD.
Recently, to realize high-precision alignment, a method of measuring movement of a magnetic head arm using an optical means in place of mechanically pressing the magnetic head arm has been proposed. FIG. 1B shows an example of a device using an optical alignment means.
Referring to FIG. 1B, the device includes a (helium-neon) laser source HeNe, mirrors M1 and M2, a beam splitter BS, a retroreflector CC like a corner cube placed at a given portion on a magnetic head arm ARM1 and a light-receiving element PD.
In this device, the laser source HeNe, mirrors M1 and M2, beam splitter BS, and retroreflector CC construct a Michelson interferometer. Interference light obtained by bringing a light beam L1 via the retroreflector CC and mirror M2 and a light beam L2 via the mirrors M1 and M2 to interference is detected by the light-receiving element PD to obtain the position information of the magnetic head arm ARM1. Based on the obtained detection signal, a signal processor SP1 generates a command and controls a current to be supplied from a voice coil motor VCMD to a voice coil motor VCM to directly move the magnetic head arm ARM1, thus achieving appropriate control.
However, in such device, the retroreflector CC like a corner cube must be placed on the magnetic head arm ARM1, thus posing problems about the space to be assured, attachment/detachment of the retroreflector, poor control characteristics due to an increase in weight, and the like.
It is an object of the present invention to provide an interference device, position detection device, and alignment device, which can detect position information of an object such as a magnetic head arm with high reliability and high precision and at high resolution without arranging any special member on the object side, can align the object without placing any specific member on the object side, and can stably measure position information of a magnetic head arm of a hard disk, whose rotational speed is becoming higher year by year, irrespective of an air flow in a hard disk housing, and a servo signal write device using them.
Other objects of the present invention will become apparent from the following description of the embodiments.